I second Tinkerbell Celebi and, if this set is chosen, another potential partner would be:
Lucario@Life Orb
4HP/252 Sp Atk/252 Spe
Inner Focus
Timid
-Nasty Plot
-Aura Sphere
-Dark Pulse
-Vacuum Wave
This thing is a BEAST! Here are some calcs (I use the Smogon damage calculator, but make a pokemon with the same stats for BW pokes. Feel free to check my calcs.)
Lucario@Life Orb +2 252 Timid Aura Sphere vs 252/0 Bold Blissey: 75.9% - 89.4%, OHKO with a bit of residual damage
Lucario@Life Orb +2 252 Timid Aura Sphere vs 252/0 Bold Eviolite Chansey: 63.4% - 75%, a clean 2HKO
Lucario@Life Orb +2 252 Timid Aura Sphere vs 252/168 RelaxedFerrothorn: 101.1% - 119.3%, OHKO
Lucario@Life Orb +2 252 Timid Aura Sphere vs 252/0 Impish Skarmory: 93.4% - 109.9%, OHKO with SR
Lucario@Life Orb +2 252 Timid Aura Sphere vs 252/224 Careful Jirachi: 42.6% - 50.5%, 2HKO with SR and 1 layer of Spikes